What is the area formula of an obtuse triangle?

1 Answer
Dec 26, 2015

The area of ANY triangle equals to half of a product of its base by its altitude. That includes triangles with an obtuse angle.
See below.

Explanation:

Consider triangle #Delta ABC#:
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Its area equals to a difference between area of #Delta ABD# and #Delta ACD#.
The first one equals to
#S_(ABD) = 1/2*BD*h#
The second one equals to
#S_(ACD) = 1/2*CD*h#

Their difference equals to
#S_(ABC) = 1/2*BD*h - 1/2*CD*h = #
# = 1/2*(BD-CD)*h = 1/2*a*h#

As you see, the formula is exactly as for a triangle with all acute angles.